Damage to the small, unmyelinated fibers in the peripheral nerves that innervate the skin and internal organs causes Small Fiber Neuropathy (SFN). This peripheral nerve disease specifically affects small diameter nerve fibers. Symptoms of SFN include a decrease in the number, density and length of small nerve fibers in the epidermis of skin biopsy specimens. It is important to diagnose SFN since detection at an early stage can predict progression to a larger-spread neuropathy.
The first step in diagnosis of SFN is studying the history of the patient and physical examination that include a detailed review of symptoms, rate of progression, and complaints suggestive of autonomic fiber involvement. Another test to diagnose SFN is the Quantitative Sensory Testing (QST). QST provides a threshold for detection of thermal sensation, thermal pain and vibratory sensation. However, QST has a few limitations such as abnormalities in either the central nervous system or peripheral nervous system can result in the same deficit, inability to distinguish between feigned and true loss of sensation to name a few.
One of the highly specific and sensitive tests for SFN is the punch skin biopsy. A punch biopsy can be taken typically from the sites of interest in evaluation of SFN. Some of the common areas of interest are the proximal and distal arm, lateral distal leg, lateral proximal thigh, lateral distal thigh and dorsum of the foot.
Samples are sectioned and stained using immunohistochemistry. However, while working with large number of patient samples, chances of introducing human errors increases as sections get transferred for each step of immunoassay. In addition, tissue damage is introduced while transferring sections rendering the immunostained sample damaged and hence unusable for examining under the microscope in order to diagnose SFN.